In most industrial fields, it is desirable to have high-performance insulation systems to ensure that fluids being conveyed in pipework are maintained at constant temperature so that transfers between pieces of equipment can be made possible over considerable distances, e.g. as much as several hundreds of meters, or even several kilometers. Such distances are commonplace in industries such as oil refineries, liquefied natural gas installations (at −165° C.), and undersea oil fields that extend over several tens of kilometers. Such oil fields are being developed in depths of water that are becoming ever deeper, and can be at depths considerably greater than 3000 meters (m).
Numerous systems have been developed for reaching a high level of thermal performance, and specific versions have been developed to accommodate great depths as appropriately as possible, i.e. to be capable of withstanding pressure at the bottom of the sea. The highest performance technologies that have been developed for achieving this objective are so-called pipe-in-pipe (PiP) technologies in which an inner pipe conveys the fluid, and an outer pipe disposed coaxially around the inner pipe comes into contact with the surrounding medium, i.e. water. The annular space between the two pipes can be filled with lagging material, or it can be evacuated so as to be free of gas.
In this type of pipe, the annular space, whether or not filled with lagging material, is generally at an absolute pressure that is lower than atmospheric pressure, and it might be completely evacuated, so to a first approximation, the inner pipe can be considered as radially withstanding the bursting pressure due to the internal fluid, while the outer pipe withstands implosion created by the hydrostatic pressure (pgh) at the sea bottom, which pressure is about 1 megapascal (MPa) per 100 m of depth of water, i.e. 30 MPa at a depth of 3000 m. The axial effect due to pressure, referred to as the “bottom” effect, acts on the circular section of the pipe and parallel to the axis of said pipe, and is shared, to a first approximation, by both pipes (since they are connected together at their ends), pro rata the respective sections of their materials, generally steel.
For installations for use at great depth, undersea pipes and undersea coaxial pipe assemblies are assembled on land to constitute elements having a unit length of the order of 20 m to 100 m, depending on the support capacity of the laying system. They are then transported in this configuration out to sea on a pipe-laying ship. During laying, the unit lengths of the various coaxial pipe assembly elements are connected to one another on board the ship progressively as laying proceeds. It is therefore important to be able to integrate making the connections in the process for constructing the pipe and laying it on the sea bottom, while slowing the process down as little as possible so that it can be performed quickly and easily.
While laying a conventional PiP in great depth, by way of comparison or as described in this patent, said PiP is subjected to bending, mainly in its bottom portion close to the sea bed. Bending is at a maximum at the point of contact with the sea bed since the radius of curvature decreases from the surface down to the point of contact with the sea bed where it is at its minimum, with the PiP thereafter resting substantially horizontally on the bottom of the sea and presenting a radius of curvature that is infinite. The bending that occurs during laying creates high levels of stress in each of the tubes of the PiP and in the connection zone between two successive lengths of PiP.
For this purpose, use is made of junction parts or connection parts that are made of forged steel and that are assembled to the ends of said coaxial pipe assembly elements for joining together. The junction part at the downstream end of a first coaxial pipe assembly element that has not yet been joined is connected to the junction part at the upstream free end of a second coaxial pipe assembly element that has already been joined at its downstream end.
Patents GB-2 161 565 and GB-2 191 842 describe a PiP and its method of assembly, and also two methods of making forged connection or junction parts.
One of the shortcomings of the junction forgings proposed in those prior patents lies in the connection zones of said junction parts, since the diameter of the parts is reduced and corresponds substantially to the diameter of the inner pipe. As a result there is a very significant change in the second moment of area “inertia” of the cross-section of the PiP between the main or intermediate zone of said PiP and said end or connection zone between two of said unit lengths of PiP, which leads to a point of weakness being created at each of these welded connections between two forgings, the zone of said welding then being particularly sensitive to fatigue phenomena, both during laying and during the lifetime of the pipe.
To avoid having such a zone of weakness and to conserve a substantially constant inertia for the cross-section, it is possible to increase the wall thickness of the forging over the entire zone situated between the solid portion of said forging and the chamfered zone where welding is performed. However it is then necessary substantially to double said thickness. For pipes of large diameter that are to be laid at great depths, welding becomes problematic because of the very great thickness of steel, since said thickness can be as great as 40 millimeters (mm) to 50 mm, thus requiring welding techniques that are very difficult to perform, and indeed in some circumstances practically impossible to perform without including defects, given the dynamic effects that can be applied to the mass of molten steel while at sea. In addition, since said welding is performed on board pipe-laying ships, which ships present extremely high hourly costs, the cost of an installation becomes prohibitive, and the risks of failure are considerable because of the complexity of said on-site welding operations.
In EP-1 771 679 in the name of the Applicant, it has been proposed to connect together unit lengths of a PiP type coaxial pipe assembly that is improved so as to facilitate implementing the connection means and the operations of making a connection, particularly by optimizing laying equipment, and in which the connection zones between two unit lengths are reinforced so that the stresses generated during laying are minimized and so that fatigue behavior for bottom-surface connections is greatly improved.
To do this, in EP-1 771 679, provision is made for a circularly symmetrical junction part for joining together two elements of an assembly of at least two coaxial pipes, the assembly comprising an outer pipe containing an inner pipe defining an annular space preferably containing a lagging material, said junction part being characterized in that it is defined as follows:                in a radial direction relative to a longitudinal axis XX′ of symmetry of said part, it is defined by a cylindrical inner wall of substantially the same diameter as the intermediate portion of said inner pipe, and by a cylindrical outer wall of diameter substantially equal to the outside diameter of the intermediate portion of said outer pipe; and        in the direction of the longitudinal axis XX′:                    on the end of said junction part that is to be connected by welding to the ends of said outer and inner pipes of a said element of an assembly of at least two coaxial pipes, said outer and inner walls of said junction part forming in longitudinal section respective outer and inner first branches of substantially the same thickness as said outer and inner pipes to which they are to be connected, said outer and inner first branches defining a first annular cavity; and            at the opposite end of said junction part that is to be joined to another said junction part, itself connected by welding to the end of another element of an assembly of two coaxial pipes, said outer and inner walls forming in longitudinal section respective outer and inner second branches defining a second annular cavity;            the bottoms of said first and second cavities being spaced apart in said longitudinal direction XX′ so as to define a solid zone of said junction part in which said outer and inner walls form the outer and inner faces of a single cylindrical wall.                        
A junction part of this type is constituted by a single metal forging, i.e. it is made as a single piece, preferably of steel, and more preferably of a steel alloy.
In EP-1 771 679 provision is also made for an assembly of at least two coaxial pipes constituted in its intermediate portion by an outer pipe and an inner pipe defining an annular space, preferably containing lagging material, the assembly comprising:                at at least one of its ends a junction part as defined above, connected thereto by welding via the ends of said outer and inner first branches and the same-thickness ends of said outer and inner pipes, respectively;        one of said inner and outer first branches of said junction part presenting an end that projects relative to the end of the other first branch by a length that is suitable for matching the length of said inner and outer pipes relative to each other at the ends of said elements of said coaxial pipe assemblies; and        said first annular cavity is preferably filled with a said lagging material.        
A method of joining together two elements of an assembly of at least two coaxial pipes, as defined above, comprises the following steps:
1) joining a said first coaxial pipe assembly element as defined above, having a first said junction part as defined above, at its downstream end to a second said coaxial pipe assembly element as defined above, having a second said junction part at its upstream end, said two outer second branches of said first and second junction parts preferably being of the same thickness; and
2) bringing together and welding together only the free ends of said outer second branches of said first and second junction parts of said two elements of an assembly of at least two coaxial pipes to be joined together.
Forming said first and second annular cavities serves firstly to establish continuity in terms of the inside diameter of the inner pipe, and secondly to provide relative continuity and identical “inertia” for the cross-section both of the intermediate or main zone of the PiP and of the connection zone, the thickness of the outer branch of the forged junction part being substantially equal to or slightly greater than the thickness of the intermediate portion of the outer pipe.
The separation between the ends of said outer and inner first branches and the bottom of the first cavity, and the separation between the end of said second outer branch and the bottom of the second cavity make it possible to perform welding under better conditions, since the masses of steel on both sides of the welding zone are substantially equal, and the welding zone is then no longer disturbed by a “radiator” effect caused by the solid and massive zone situated between the bottoms of said first and second cavities, where said disturbance consists in unbalanced cooling between the left and right sides of the welding zone.
Finally, the continuity in the diameter of the outer wall along said junction part and relative to the outside diameter of the intermediate portions of the outer pipes makes it possible to create a considerable increase in the “inertia” of the cross-section in the connection zone between two adjacent junction parts, thereby reinforcing the connection, precisely where stresses are at a maximum. The “inertia” of the cross-section of a pipe about its center varies with the fourth power of its radius, which leads to a considerable thickness being necessary in the prior art as described in GB-2 161 565 or GB-2 191 942. In contrast, when the cross-section in question is that of the outer pipe of the PiP, the thickness required is greatly reduced, and even under certain circumstances divided by two, thereby considerably simplifying assembly operations by welding performed on board installation ships under conditions that can be difficult.
Furthermore, the fact that the two adjacent junction parts are welded together only via the ends of said outer second branches, enables all of the load transfer and stress phenomena to be localized in the outer level without involving said inner walls, thereby making it possible to have better control over the risks of cracking or fatigue phenomena and to avoid the device collapsing completely at the inner wall.
In addition, the fact that the two ends of said inner second branches of two joined-together adjacent junction parts of the invention are not welded together, serves to accommodate small movements between said facing inner walls due to possible bending or variations in pressure or temperature, and allows said inner walls to deform plastically, since it is possible for said inner second branches to be upset without any risk of transferring contact compression loads, thus making it possible to avoid disturbing the distribution of stresses in the assembly zone with the major portion of the stresses being transferred via the outer walls of said parts.
The formation of said cylindrical inner wall that ensures almost complete continuity with the inner pipe makes it possible to avoid vortex type turbulence phenomena in the flow of fluid inside the device after elements have been joined together and on going past the join between two said junction parts of two adjacent portions of PiP.
All of these characteristics contribute to greatly improving behavior in bending, and also in fatigue, of a device involving two elements of a coaxial assembly fitted with said junction parts and connected together on board installation ships.
Furthermore, said junction parts can be manufactured and connected relatively easily and reliably both concerning joining together two adjacent junction parts, and concerning the connection of a junction part with one end of an assembly of at least two coaxial pipes.
In an advantageous embodiment of EP-1 771 679, said inner second branch includes on its end face at its free end and extending in the longitudinal direction, a male or female centering element suitable for co-operating with a female or male element at the free end of a said other inner second branch of another said junction part to which it is to be joined, in such a manner as to:                provide a centering effect between two junction parts when they are brought together in order to be joined together; and        adjust the spacing e, where e is equal to 1 mm to 5 mm, preferably 2 mm to 4 mm, between the end faces of said outer second branches of said junction parts to be joined together while they are being brought together for joining purposes so that, preferably, joining can be performed by welding and said welding can be implemented over the entire thickness of said end faces of said outer second branches that are to be joined together.        
In another advantageous embodiment, in EP-1 771 679, the thickness of said inner second branch of one of the two junction parts joined together end-to-end tapers between the bottom of said second annular cavity and said end face of said inner second branch, the surface of said inner second branch thus being inscribed in a frustoconical envelope, and said inner second branch presents reduced stiffness relative to the inner second branch of the other junction part to which it is joined.
This inner wall that tapers in said inner second branch can act as an optional abutment while facilitating plastic deformation and potential upsetting during bending movements or variations in pressure or temperature, with the major fraction of the stresses being transmitted almost completely via said outer second branch.
As a result of this, in EP-1 771 679, after the two junction parts have been joined together, said second cavity is not sealed off from the inside of said inner wall and said internal pipe, since when a fluid begins to flow inside it, the fluid migrates into the second cavity, with overall sealing of the PiP pipe being provided by the outer weld between the ends of said outer second branches, so the fluid becomes trapped in said second cavity throughout the lifetime of the installation.
In order to obtain an optimum centering effect, EP-1 771 679 describes an embodiment in FIG. 2E in which the surfaces of said two inner second branches co-operating with each other by making contact during joining of the two junction parts are inscribed in a common frustoconical envelope, said two inner branches co-operating by making contact having the same “inertia” and thus the same stiffness, with their contact surfaces during assembly and after welding of the outer second branches coinciding with their overlap surfaces along the longitudinal axis XX′, which is at least greater than half the length of said inner second branches.
Because of this relatively large-area contact surface between said two inner second branches, coinciding with their overlap zones, the initial elastic deformation stress in said metal-on-metal contact surface is insufficient and/or not uniform over the entire surface, thereby giving rise to leakage paths and to a lack of leaktightness for said second cavities, in that embodiment also.